One problem inherent to buying books online is that you aren't always sure of their content. Google Books and previews help with this, but they aren't of all books. Sometimes you get a book and realize that it just isn't what you wanted (Oh... this book on Ruby assumes you're using 1.8.6, etc).
If the seller has a return policy that accepts it in that time, and its in good condition, then the retailer should accept it back. This is how I view it to work on Amazon anyway. Other places like eBay, it doesn't work like that, but it is known.
Order a pair of shoes from Zappos. Get them. Look at them, and then send them a return note saying, "They are exactly as I wanted, and fit perfectly, but I just realized I don't need them", and you will not get a return note about how you might have 3D scanned them.
Overall, I really feel that if you're going to sell in Amazon's house, then you've gotta act like Amazon. If not, then you should consider selling elsewhere.
As an aside, in some juristictions, customers are legally entitled to a full refund if they buy something online and within 7 days, decide they don't want it.
The real problem is that merchants can't state that they don't accept refunds in cases where people bought and received what they asked for but they didn't actually want the think they bought. Kind of a corner case I'd think, but one that led to the disappointment of both people in this transaction.